Prop Talk (maybe this should have its own section)

[gold-n-rod]

How do two props of the same design (same make and model) and pitch behave differently when there is a 1" diameter difference between the two.

For example, a 13.25 x 21P versus a 14.25 x 21P? What performance differences would one expect to find?

Thanks in advance.

[blackhawk]

A smaller diameter prop will work well on a smaller boat. It will accelerate harder and be turned easier with less hp. A larger diameter prop will carry a larger boat better and require more power to turn. On the same boat a smaller diameter prop will turn higher rpm. I am by no means an expert but I have learned a lot about props recently(the hard way!). :rolleyes:

Harbormaster, not a bad idea about having a prop section hah? :D

[EricG]

As it was explained to me by a local prop guy, the pitch is a theoretical number based on how far the prop would travel through a solid in one revolution (most of us already knew that).

But, the diameter is just another attribute of the pitch value. In other words, a 25" mirage will have a different diameter than say a 25" Turbo due to different blade design, but a 25" Mirage can't have a different diameter than another 25" mirage, or one of them isn't a 25. If props have been worked on, well, then this obviously could cause the diameters to be different, but I maintain that the pitch rating would actually be different also.

BTW - my limited experience with prop testing has shown that Blackhawk's response is correct, I could spin a 21" stilletto a lot better than a Mirage+ 21" on my old Z. The stilletto was a much smaller diameter prop, but they both had the 21" rating.

I'm just saying that I don't believe you can buy multiple diameters of the same prop design and pitch... But, I'm sure someone will prove me wrong.

I too vote for adding the "prop talk" section to the board. Although that will cut the performance section in half :D :D

-EG

[gold-n-rod]

I'm just saying that I don't believe you can buy multiple diameters of the same prop design and pitch... But, I'm sure someone will prove me wrong.

Turbo offers both 13.25 and 14.25 versions of its Turbo I prop across the pitch range.

So then, is a smaller prop (of the same pitch) easier to spin because the blades have less area, because it weighs less, or what??????????????

The longer I live, the more I realize I don't know.

[Sam]

It's easier to spin because it is a smaller prop I.E. 14.5" vs. 13.5".

Sam

[EricG]

Turbo offers both 13.25 and 14.25 versions of its Turbo I prop across the pitch range.

So then, is a smaller prop (of the same pitch) easier to spin because the blades have less area, because it weighs less, or what??????????????

The longer I live, the more I realize I don't know.

Well...that pretty much shoots my theory away then. Although to my oversimplified brain, they must have slightly different blade design to make up for the diameter change. I would love to see a back to back test on the same boat to see what the difference in performance is...

EG

[gold-n-rod]

I feel like the ignorant leading the clueless here (no offense to any contributer) but what makes it smaller? Obviously the diameter is less, but does that mean the blades are shorter (or smaller) or are they the same size with a different orientation or dimension? And, to further confuse matters, does it weigh proportionally less?

Take 2 men. One is 6" taller than the other. Physically, can they do the same amount of work? Physical condition being equal, doesn't it depend on their weight, how it is distributed, the length of their arms, legs and torso?

Damn, now I've confused myself..... :bonk:

[hyperlite98]

There is less resistance / drag in the water for the 13.25 vs. the 14.25 so it should be able to spin faster givin the same power application.

i think ;)

JT

[Rootsy]

inertia my friend... inertia... think Newton and his o so famous laws...

[mattyboy]

ahh grasshopper let's not forget
blade number , shape ,thickness and design
rotation
and salt or fresh
and what rpm your motor makes ponies
all little factors of prop selection
but on a 16 I would go with what Jamie says :) ;)

[blackhawk]

There is less resistance / drag in the water for the 13.25 vs. the 14.25 so it should be able to spin faster givin the same power application.

i think ;)

JT

Yep, and also MORE BITE with the bigger prop. Smaller diameter props are usually used on smaller boats with lower hp. 13.25 is pretty small.

[gcarter]

Propellor design is highly imperical. Dave Gerr N.A. in his "Propellor Handbook" , in one of the first chapters, uses an example of several manufacturers propellors, all having the diameter and pitch, having absolutely nothing in common with each other, other than diameter. :bonk:

:wavey:

[BigGrizzly]

GCarter is correct The example of Turbo vs Marage plus is like apples and bannas not even close. The progression of the blade is totaly different. what you will find that som makeres have the same prop with a smaller Diameter bit that also has a smaller hub for a smaller gear case- ob vs IO

[blackhawk]

I 100% agree but he's comparing a Turbo 1 to a Turbo 1!

[DonCig]

After reading this thread I have the following question to ask.
If prop A is 15" in dia and is a 26" pitch and the boat can turn 4,800 rpm WOT. And prop B is from the same manufacturer and has the same blade design and the only thing different about prop B is that it is 14.5" in dia and the boat can turn 5,000 rpm WOT.

And the recomended rpm range for the engine is 4,800 rpm to 5,000 rpm.

Wouldn't the better prop be the one that had the lower slip ratio?

DonCig

[gcarter]

After reading this thread I have the following question to ask.
If prop A is 15" in dia and is a 26" pitch and the boat can turn 4,800 rpm WOT. And prop B is from the same manufacturer and has the same blade design and the only thing different about prop B is that it is 14.5" in dia and the boat can turn 5,000 rpm WOT.

And the recomended rpm range for the engine is 4,800 rpm to 5,000 rpm.

Wouldn't the better prop be the one that had the lower slip ratio?

DonCig
That would be my opinion.

[Wild Man]

Ok newbe here.

I would use the prop with less slip, unless the one with more gave a better hole shot.

That in mind how do you go about slecting a prop? What about 3 blade ver 4 blade?

I could us some help as I just picked up a mint (68hrs) 1998 21 LSR with the 350 mag motor. It has a turbo 14.25x17, Motor tag recomends 4400-4800 r.p.m.
With me and the wife, 50 gals of fuel I can trim it out and turn 5100 r.p.m.

If I go with more pitch do I lose hole shot? What about a 4 blade?

Wild Man

[vanstee]

88 ragazza 3.0L

15.25 x 15 got me 3000 RPM
13.5x15.25 allowed me to cavitate the prop.
Just got it back from having alittle more cup put in it. will advise.

[blackhawk]

Great reply Scott! Don't get caught up in the "bench racing" of prop selection. This is not a science, it's an art! Like Scott said which prop gave you the best speed?